sol_041.md

Middle of the Linked List - Practice - LeetCode

Given the head of a singly linked list, return the middle node of the linked list.
If there are two middle nodes, return the second middle node.

Example:1

Input: head = [1,2,3,4,5]
Output: [3,4,5]
Explanation: The middle node of the list is node 3.

Example:2

Input: head = [1,2,3,4,5,6]
Output: [4,5,6]
Explanation: Since the list has two middle nodes with values 3 and 4, we return the second one.

Solution 1

Time - O(N)
Space - O(1)

class Solution {
public:
    ListNode* middleNode(ListNode* head) {
        int len = 0;
        ListNode* curr = head;
        while(curr){
            curr = curr->next;
            len++;
        }
        
        len = len/2;
        curr = head;
        while(len--){
            curr = curr->next;
        }
        return curr;
    }
};

Solution 2

Time - O(N)
Space - O(1)

class Solution {
public:
    ListNode* middleNode(ListNode* head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast && fast->next){
            fast = fast->next->next;
            slow = slow->next;
        }
        return slow;
    }
};